For the gamers out there, Valve software's "Steam" gaming client package is available. Similar to the iTunes store, Steam is a single program through which you can purchase, download, and manage games for your computer. In addition, the program offers networking options for locating and joining online game servers, chatting, and providing in-game communication enhancements.
Valve offers over a thousand games through Steam, and for OS X the debut will see Left 4 Dead, Left 4 Dead 2, Team Fortress 2, Counter-Strike: Source, Portal, and the whole Half-Life series available. Many of these games are run using Wine-like compatibility layer for OS X, which has been seen in applications like Codeweaver's "Crossover" and "Crossover Games" packages, which offer very high performance solutions to porting Windows games to OS X and other Unix-like operating systems.
The client is available for free from the following location, and is a simple drag-and-drop installation to your Applications folder: http://store.steampowered.com/public/client/steam.dmg
On Windows, the system requirements are a 1GHz processor with at least 512MB RAM, and on the Mac you will need at least an Intel Mac with similar specifications; however, individual games will have their own system requirements and many people with Intel GMA graphics on their systems (notably Mac Mini and MacBook owners) will not be able to run graphically intensive games.
Currently, while the client is available, the services are still restricted to those using the beta version, so you will not be able to log in. Given the availability of the official client, this should change soon. Happy Gaming!